BEIRUT, Sept 20 (Reuters) - At least 54 people were killedwhen a Syrian air strike hit a fuel station in the northernprovince of al-Raqqa on Thursday, an area of heavy fightingbetween government and rebel forces, a British-based monitoringgroup said.

The Syrian Observatory for Human Rights, which has a networkof activists across Syria reporting on government violenceduring the 18-month-old revolt, cited an activist in the regionsaying that more than 110 people were dead or wounded.

A video published by activists, said to be from al-Raqqa,showed black clouds of smoke rising from the wreckage of thepetrol station as bewildered residents examined the scenefollowing the attack by a Syrian air force jet.

Government forces shelled rebels near a border crossing withTurkey some 30 km (18 miles) away on the northern fringes ofal-Raqqa, a day after it was seized by the insurgents.

A Reuters witness on the Turkish side of the border heardheavy gunfire and explosions close to the Tel Abyad border post,where an opposition flag still fluttered. Residents rushedtowards the border as the gunfire intensified.

It was impossible to verify the authenticity of theactivists' video, and most foreign journalists are barred entryinto Syria, making varying accounts of events difficult toconfirm.

President Bashar al-Assad has used helicopters and fighterjets to fire at and bomb parts of the country where insurgentshave been operating, including residential districts of thecapital and Syria's main cities.

Assad's forces have targeted petrol stations in rural townsand villages and along main roads to deprive rebels of fuel.Civilians have set up smaller, discreet fuel outlets.

Activists say more than 27,000 people have been killed in aconflict that began with peaceful street protests and mushroomedinto civil war after Assad tried military force to stamp out theunrest. Last month was the bloodiest yet.

Earlier on Thursday, Syria's information ministry said thata Syrian military helicopter that crashed near the capital hadclipped the tail of a Syrian Arab Airlines passenger plane, butthe 200 people on board escaped unharmed.

"The helicopter struck the tail of the plane ... The controltower at Damascus airport confirmed that the plane landed safelyat Damascus airport and all 200 passengers are in good health,"a statement published on the state news channel Syria TV said.

REBELLIOUS DAMASCUS DISTRICT RAIDED

On the ground, security forces surrounded and raided arebellious southern district of Damascus, arresting more than100 people, and activists said several others were shot dead.

An opposition activist called Abu Salam, who lives in theYarmouk district where rebels have been hiding out in recentdays, told Reuters that many residents were trapped.

He said tanks and soldiers had sealed all the entrances andhundreds of soldiers were searching the area on foot and ontrucks mounted with heavy machineguns.

"We are hiding in our homes. I am afraid to leave the houseso I am sitting here waiting to see if they reach my street, ifI will be arrested or shot dead," he said, adding that at leastthree people, two men and a young women, were shot dead whensoldiers saw them running out of a park on Thursday morning.

He said another five rebels found hiding were executed.

A resident who toured Yarmouk a day earlier said rebelfighters, who have been flushed out of many surroundingdistricts, had moved into a southern section of the district andcome under intense army bombardment overnight.

Assad has long maintained that foreign-backed militants havebeen leading the revolt, which started with peaceful protestsbut turned violent as the army fired on demonstrations androunded up thousands.

State media said on Thursday that soldiers had killed 100Afghan "terrorists" in the northern city of Aleppo. Rebels inthe area said that report was government propaganda, saying thatthe district of Bustan al-Qasr - where the attack supposedlytook place - has not been entered by Assad's troops.

DIVIDED WORLD CANNOT HALT VIOLENCE

Global and regional powers have failed to halt the violence.Iran and Russia have backed Assad while the United States andEuropean allies want him toppled but shrunk from intervening ina conflict steeped in ethnic and sectarian rivalries that couldspill over Syria's borders and inflame the wider Middle East.

Iraq denied on Thursday a Western intelligence report thatsaid Iranian aircraft and trucks had transported weapons andmilitary personnel through Iraq to Syria to help Assad corralthe uprising and Belarus denied trying to sell weapons to Syria.

"Iraq has confirmed that it will never be involved orhelping or allowing any shipment via its air space or land toSyria," Iraqi government spokesman Ali al-Dabbagh told Reuters.